Method of joining electronic components to a substrate

ABSTRACT

A method for the temporary fixing of an electronic component (10) having elevated contact metallizations (11) to a substrate (17) provided with terminal surfaces (16) for a subsequent thermal connection of the contact metallizations to the terminal surfaces, wherein a bonding agent is applied to the component and/or the substrate and wherein the bonding agent used is an alcoholic liquid medium containing an alcohol (13) whose surface tension is used to form bonding forces between the contact metallizations and the terminal surfaces and whose boiling point is below the melting temperature of the contact metallizations (11).

The present invention relates to a method for the temporary fixing of anelectronic component having elevated contact metallizations on asubstrate provided with terminal surfaces for a subsequent thermalconnection of the contact metallizations to the terminal surfaces,wherein a bonding agent is applied to the component and/or thesubstrate.

Methods of the type referred to in the introduction are used for examplein order to obtain a detachable mechanical preliminary fixing inso-called flip-chip contacting. In this contacting method the chip isapplied to the terminal surfaces of a substrate with its elevatedcontact metallizations, also referred to as bumps, facing downwards. Theactual connection between the chip and the substrate is then produced bysubjecting the bumps to thermal energy whereby a remelting of the bumpsoccurs. To fix the relative positioning between the chip and thesubstrate prior to the remelting, it is known to provide an applicationof fluxing agent between the bumps and the terminal surfaces, whichfluxing agent establishes a bond between the chip and the substrate forsuch time as the bumps have not yet melted. When the bumps are in themolten state, the fixing action of the fluxing agent ceases andtherefore the chip is mounted on the substrate in quasi-floatingfashion. Here a self-aligning method can occur which is brought about bythe surface tension of the contact metallization material and whichprovides for an exact positioning of the chip on the substrate.

In the above described method for temporary fixing, this advantageousexploitation of the self-aligning effect is acquired together with thedisadvantage that anionic or cationic fluxing agent residues remain onthe remelted bumps even after the formation of the connection betweenthe chip and the substrate. This frequently has a disadvantageous effecton the reliability of the provided connection.

Another known method for temporary fixing consists in that, before theproduction of the final connection between chip and substrate, apressure welding connection is effected between the chip bumps and thesubstrate terminal surfaces, said pressure welding connection beingreleased again when the subsequent thermal connection is established,whereby here again the advantageous self-aligning effect is facilitated.However, the high mechanical stress upon the chip associated with thepressure welding proves disadvantageous as in this method relativelyhigh reject quotas due to mechanical damage to the chips are noted.

Therefore the object of the present invention is to propose a methodwhich facilitates a temporary fixing of electronic components, such asfor example chips, on a substrate without entailing the disadvantageousaccompanying phenomena described in the foregoing.

This object is fulfilled by a method comprising the features of claim 1.

In the method according to the invention, the bonding agent selected isan alcoholic liquid medium containing an alcohol whose surface tensionis used to form bonding forces between the contact metallizations andthe terminal surfaces and whose boiling point is below the remeltingtemperature of the contact metallization.

When an alcoholic medium of this kind is employed, which can be appliedto part of or all of the surface of the substrate or component, use ismade of the property of many alcohols of forming--by virtue of theirhigh interfacial or surface tensions--correspondingly high surfaceforces in the contact region between contact metallization and terminalsurface, these resulting in high bonding forces. Use is also made of theadvantageous property of alcohol of evaporating with no residue so that,in contrast to the methods employing conventional fluxing agents, noanionic or cationic residues, which disadvantageously influence thereliability of the connection, remain on the remelted contactmetallizations. The setting of the boiling point below the remeltingtemperature of the contact metallization ensures that on the one handthe bonding forces are effective until the remelting of the contactmetallization and on the other hand after the remelting of the contactmetallization the alcohol used as bonding agent is eliminated withoutresidue.

The method according to the invention naturally can also be appliedadvantageously when the elevated contact metallizations are arranged onthe substrate or are arranged both on the component and on thesubstrate.

In accordance with a preferred variant of the method, the bonding agentis applied only to the elevated contact metallizations, whichfacilitates a specially dosed use of the bonding agent and also obviatesthe need for possibly required cleaning of any bonding agent residuesfrom the remaining surface of the electronic component and/or substrateafter the production of the thermal connection.

In accordance with claim 3, the method can be employed in the case ofso-called face-down contacting of the component. Such contacting iscarried out for example in the known flip-chip contacting method. If thebonding agent is applied for this purpose to the contact metallization,the method can be carried out without special preliminary treatment ofthe substrate terminal surfaces.

The alcohols which have sufficient surface tensions to form the bondingforces between the contact metallizations and the terminal surfaces aregenerally alcohols which have long molecular chains and therefore tendto be high-boiling and possess a relatively high viscosity. Therefore itproves particularly advantageous if, in addition to a relativelyhigh-boiling alcohol for the formation of the above explained bondingforces between the contact metallizations of the component and theterminal surfaces of the substrate, the bonding agent also contains asolvent in order to permit the application of the bonding agent in aparticularly thin layer.

If a relatively low-boiling alcohol is used as solvent or diluent, it isensured that the inert effect of the alcohol is not negatively affectedby the addition of the solvent.

Relatively high-boiling alcohols, such as glycol, glycerol, pyrolydol orother alcohols having a correspondingly high interfacial tension proveparticularly suitable for use in the method according to the invention.

Relatively low-boiling alcohols, such as methanol, isopropanol or otheralcohols having a correspondingly low interfacial tension and lowboiling point prove particularly suitable for use as solvent in themethod according to the invention.

In the following the method will be explained in detail in the form of amethod variant, which is to be considered as an example, makingreference to the drawings wherein:

FIG. 1 illustrates a chip whose surface is provided with a bondingagent;

FIG. 2 illustrates the application of the chip shown in FIG. 1 to asubstrate in the flip-chip method;

FIG. 3 illustrates the remelting of the chip bumps for connection toterminal surfaces of the substrate.

FIG. 1 shows a chip 10 with a plurality of elevated contactmetallizations 11 referred to as so-called bumps. The contactmetallizations 11 are provided with a bonding agent solution 12consisting of high-boiling alcohol 13 with long molecular chains andlow-boiling alcohol 14, which solution can be applied for example indrop form to the contact metallizations 11.

The film-like coating 15 in FIG. 1, formed from the high-boiling alcohol13, on the contact metallizations 11 remains behind when the low-boilingalcohol 14 serving as solvent has evaporated at room temperature.

The contact metallizations 11 shown in FIG. 1 can be formed for examplefrom an eutectic gold/tin alloy having a solidification temperature of278° C. Glycerol, with a boiling point of 286° C., is a suitable choicefor the high-boiling alcohol 13 which remains on the contactmetallizations 11 as coating 15 after the evaporation of the solvent.Methanol is suitable for use as solvent.

As shown in FIG. 2, for contacting on a substrate 17 provided withterminal surfaces 16, the chip 10 is applied to the terminal surfaces 16of the substrate 17 with the contact metallizations 11 facing downwards,as customary in the flip-chip method. The drying of the coating 15 givesrise to surface forces which produce a fixing bond between the chip 10or the contact metallizations 11 and the substrate 17 or the terminalsurfaces 16.

After the formation of the bonding forces between the contactmetallizations 11 and the terminal surfaces 16, the contactmetallizations 11 are heated, for example by subjecting the contactmetallizations to infrared radiation, to a temperature of approximately300° C., which leads to a remelting of the contact metallizations 11, asshown in FIG. 3. Before or when the remelting temperature of the contactmetallization 11 is reached, the coating 15 shown in FIG. 2 evaporates.Therefore the chip 10 can automatically and unobstructedly align itselfvia the surface forces active in the molten contact metallizations 11,the relative position now reached by the chip being as it were frozenupon the solidification of the remelted contact metallizations. As thecoating 15 evaporates at the latest when the contact metallizations 11assume the molten state, in contrast to conventional fixing using afluxing agent the surface of the contact metallization 11 is free of anyresidues of the bonding agent after the solidification.

The following may also be mentioned as further examples of suitablecombinations of contact metallizations and high-boiling alcohols: glycol(boiling point 197° C.) in the case of a eutectic tin/lead alloy (Sn/Pb63/37, solidification temperature 183° C.) and pyrolydol and glycerol inthe case of contact metallizations with a high lead content.

We claim:
 1. A method for temporarily affixing an electronic component having elevated contact metallizations to a substrate provided with terminal surfaces, for a subsequent thermal connection of the contact metallizations to the terminal surfaces, said method comprising the steps ofapplying to the terminal surfaces a mixture comprising an alcohol having a high boiling point and an alcohol having a low boiling point; contacting the contact metallizations with the terminal surfaces; evaporating from the mixture the alcohol having a low boiling point and forming a bonding coating from the alcohol having a high boiling point and affixing the elevated contact metallizations of the electronic component to the terminal surfaces of the substrate with the bonding coating.
 2. A method for temporarily affixing an electronic component having elevated contact metallizations to a substrate provided with terminal surfaces, for a subsequent thermal connection of the contact metallizations to the terminal surfaces, said method comprising the steps ofapplying to the elevated contact metallizations a mixture comprising an alcohol having a high boiling point and an alcohol having a low boiling point; contacting the contact metallizations with the terminal surfaces; evaporating from the mixture the alcohol having a low boiling point and forming a bonding coating from the alcohol having a high boiling point and affixing the elevated contact metallizations of the electronic component to the terminal surfaces of the substrate with the bonding coating.
 3. The method according to claim 2, wherein the contacting step is accomplished by positioning the electronic component above the substrate with the elevated contact metallizations of the electronic component facing downward and wherein the mixture comprising an alcohol having a high boiling point and an alcohol having a low boiling point is applied in the facing downward position.
 4. The method according to claim 1, wherein the mixture of the application step also includes a solvent.
 5. The method according to claim 4, wherein the solvent is a second alcohol having a low boiling point.
 6. The method according to claim 3, wherein the alcohol having a high boiling point is selected from the group consisting of glycol, glycerol, and pyrolydol.
 7. The method according to claim 3, wherein the alcohol having a low boiling point is selected from the group consisting of methanol and isopropanol. 